From the past, in a valve gear configured to drive, for example, an intake valve and an exhaust valve of an engine with a cam shaft, there is a known variable valve actuation mechanism that can change lift characteristics of the valves. As an example, the variable valve actuation mechanism disclosed in Patent Document 1 includes a driven member which is operated by a driving cam of a cam shaft, a pivot member which pivots in association with operation of the driven member to open and close an intake valve, and a mechanism which changes a relative positional relationship between both of the members in association with rotation of a control shaft.
An electric motor is connected to the control shaft via a worm gear mechanism. When the control shaft is rotated by the electric motor, the relative positional relationship between the driven member and the pivot member is changed, resulting in a change in a pivoting range of the pivot member. In this manner, a lift amount and a lift timing of an intake valve are changed. The electric motor operates in response to instructions from a controller, and a rotation angle of the control shaft which pivots in association with the operation of the electric motor is detected by an angle sensor and is fed back to the controller.
In order to restrict the rotation angle of the control shaft to fall within a predetermined range, a stopper is disposed on an operation track of a worm wheel fixed to the control shaft. In regard to the stopper, an elastic body such as rubber is interposed between a cylindrical contact member and a shaft member which supports the contact member, and the contact member is displaced according to deformation of the elastic body when the worm wheel comes into contact with the stopper. This loosens the engagement between threads and teeth in the worm gear and, also after that, the worm gear mechanism comes to operate smoothly.